Reduce reliance on vendored libraries (libcurl, libyaml, libftdi)

[hnez]

Hi,

I would like to package bcu for meta-lxatac linux-automation/meta-lxatac#93 (a Yocto based operating system for the LXA TAC, an embedded linux development tool - but I digress the details don't matter too much here).

It would be great for packaging if bcu did not use vendored (as in git submodules) libcurl and libyaml and would instead use the libcurl and libyaml provided by the operating system.

I tried to prepare a pull request that does that, but quickly noticed that I do not know how to do this correctly for macOS and Windows builds, so I decided against doing that and to instead open this issue so someone with more macOS and Windows building experience can hopefully take over.
I also noticed that CMakeLists.txt also contains references to ${PROJECT_SOURCE_DIR}/libftdi/src which does not seem to exist (anymore?).